All income is taxable, it maybe a revelation but true. Many times I’ve heard people say they don’t think they have to submit a self assessment and quote numbers, these are the rules from HMRC.
“If you are an employee or a pensioner and already pay tax through a PAYE code, you can sometimes ask for tax that you owe on income, such as savings and property, to be collected through your code number. You'll need to complete a tax return instead if the income you receive is:

£10,000 or more from taxed savings and investments

£2,500 or more from untaxed savings and investments

£10,000 or more from property (before deducting allowable expenses)

£2,500 or more from property (after deducting allowable expenses)

If you don't pay tax through a PAYE code you’ll need to complete a tax return if all of the following apply:

you have income to declare, for example income from savings, trusts or abroad, rental income from land or property

your total income exceeds your total allowances and reliefs

you have tax to pay on this income”

So who or when do you need to submit a self assessment?

Self-employed

Company director, minister, Lloyd's name or member

Annual income is £100,000 or more

Income from savings, investment or property

Claim for expenses or reliefs

You or your partner receive Child Benefit and your income is over £50,000

The new High Income Child Benefit tax charge, introduced on 7 January 2013, may mean you need to complete a Self Assessment tax return for the first time. You must complete a tax return if all of the following apply:

your income is over £50,000 a year

you live with a partner and your income is higher than theirs

you or your partner are entitled to receive Child Benefit (or get an equivalent amount from someone who claims Child Benefit for a child who lives with you)

you jointly decide to keep receiving Child Benefit and pay the new tax charge

Over 65 and receive a reduced age-related allowance

Receive get income from overseas

Income from trusts, settlements and estates

Capital Gains Tax to pay

Lived or worked abroad or aren't domiciled in the UK

Trustee

Don’t forget that if you are submitting a paper self assessment it must be received by HMRC by midnight on the 31 October. There is talk this is being phased out, so watch the press for updates.

The onus is on the taxpayer to inform HMRC, it’s not when they’re caught. If HMRC catches up with a taxpayer they are less likely to be sympathetic to their plight